1. Field of the Invention
The present invention relates in general to an elastic rotational coupling interposed between two rotating members for power transmission therebetween. More particularly, this invention is concerned with a fluid-filled elastic rotational coupling for flexible connection of two rotating members so as to effectively minimize the transmission of vibrations therebetween.
2. Discussion of the Prior Art
An elastic rotational coupling is known as a coupling device interposed between a driving rotary member and a driven rotary member so that a rotary motion is transmitted from the driving rotary member to the driven rotary member through the coupling device, while preventing input vibrations from being transmitted between the two members. For example, an elastic rotational coupling is disposed between a steering shaft of a vehicle and an intermediate shaft fixed to a steering gear box of the vehicle, or used as a coupling for a propeller shaft of the vehicle.
Laid-open Publication No. 51-143158 of unexamined Japanese Patent Application discloses an example of such elastic rotational coupling, which has a first sleeve fixed to a driving rotary member, a second sleeve fixed to a driven rotary member, and an elastic body which elastically connects the first and second sleeves, so that the input vibrations are damped based on the elasticity of the elastic body.
In a known elastic rotational coupling which utilizes only the elasticity of the elastic body to provide a vibration damping or isolating function, it is difficult to formulate or tune the elastic body so as to exhibit an optimum vibration damping or isolating characteristic, since the elastic body also functions as a medium through which a torque is transmitted between the driving and driven rotary members. More specifically, the vibration damping or isolating characteristic of the elastic rotational coupling is determined primarily by the dynamic spring constant of the elastic body. This means that the dynamic spring constant of the elastic body should be relatively low to improve the vibration damping or isolating characteristic. On the other hand, the power or torque transmitting characteristic of the coupling is affected by the static spring constant of the elastic body. This means that the static spring constant of the elastic body should be relatively high to improve the power transmitting characteristic or response. Since the dynamic spring constant and the static spring constant usually have a given relationship, it is not possible to lower only the dynamic spring constant. Thus, there has been a need of providing an elastic rotational coupling which exhibits an sufficiently high vibration damping or isolating effect while assuring an excellent power transmitting characteristic or response.
In particular, it is noted that the dynamic spring constant of an elastic body is generally larger than the static spring constant and tends to increase with an increase in the frequency of the input vibrations. Consequently, in the case of a steering coupling interposed between a steering shaft and an intermediate shaft which are respectively fixed to a steering wheel and a steering gear box of a vehicle, for example, there is a problem that input vibrations such as a shimmy having a frequency of about 10-40 Hz cannot be sufficiently damped or isolated, if the static spring constant of the elastic body is set high enough to permit the elastic body to smoothly transmit a rotary movement of the steering wheel to the steering gear with a high response.